So why not a assembler debugger, programmed in Java (so it can run on any platform you can imagine) , with all the nice features of a windows program

Since what you have stated here seems to indicate you have some talent in PC programming, why do you not address your talents to something that REALLY need addressing such as e.g. buy one of the remaining CDs of CodeWright and try to have an editor as good or better before CodeWright disappears (see thread about CodeWright and Borland). Also make that editor in such a fashion that it can seemlessly replace the inferior editors in the IDEs.

as to your project:

1) I do not know any professional that would use an assembler that was not included with a C compiler. Assembly coding these days is mainly "the little routine in a C project that need be optimal". Thus an assembler that is not part of a compiler/assemble/linker package is virtually worthless so WHO GIVES A HOOT?.
2) what you call a debugger must be a simulator. You will find that the existing popular simulators (such as Keil) have an ever growing library of peripheral simulators, how are you going to handle that. If you do not have a library including serial input IIC, SPI, A/D and more WHO GIVES A HOOT?
3) "any platform" is nice had it not been for the fact that all '51 developers are forced to run Windows because thing like ISP utilities, JTAG programmers, PROM burners only come with Windows software so WHO GIVES A HOOT?

In other words you are about to reinvent the wheel and
1) It does not need be reinvented
2) If so the new should be better and "Java" does not mean it is better, just slower.
3) No C compiler as part of the package = no use.

Erik